This invention relates to an active matrix display device comprising sets of row and column conductors, an array of picture elements each comprising a capacitive display element and a switching device connected to a row conductor and a column conductor, and drive means for driving the picture elements comprising a row drive circuit for applying switching signals to the row conductors, a column drive circuit connected to the column conductors for applying data signals to the column conductors, and means for supplying a digital picture information signal to the column drive circuit from which said data signals are derived.
An active matrix display device of this kind, and in which the display elements comprise liquid crystal display elements, is described in EP-A-0391654.
Display devices having column drive circuits operating with digital Picture-information video, signals can offer advantages over those operating with analogue video signals, particularly in certain applications such as data-graphic display apparatus. The digital video signals can be obtained by digital video processing circuits which generally are capable of greater flexibility than their analogue counterparts. Digital video signals could be supplied for example from a RAM store of a computer or, alternatively, provided by converting analogue TV video signals into digital form.
The display device described in the aforementioned specification includes a TFT liquid crystal panel of conventional type having a row and column array of picture elements which each include a TFT and which are addressed via sets of row and column conductors with selection signals being applied to each row conductor in turn so as to turn on the TFTs of the picture elements associated with that row conductor whereby data signals on the column conductors are transferred to respective display elements.
In the column drive circuit of the display device described in the aforementioned specification, the digital video signals are converted into analogue (amplitude modulated) data signals and these analogue data signals are applied to the column conductors of the display panel, and thence to the display elements via their TFTs to provide the analogue voltages necessary for operating the liquid crystal display elements. The amplitude of this analogue voltage determines the display effect, e.g. grey scale, produced by the display elements. The digital to analogue conversion in the column drive circuit involves translating multi-bit digital signals into pulse width modulated pulse signals, e.g. pulses whose widths are determined by the multi-bit digital signals, which are then used to sample a time-varying reference voltage so as to obtain output voltages, constituting the data signals, whose amplitudes are dependent on the durations of the time dependent signals.
Although capable of operating with an input digital video signal, this display device suffers from a number of disadvantages. The column drive circuit is not truly digital but comprises a mixture of digital and analogue circuitry. The analogue part can be expected to impose limitations on the performance of this circuit. Moreover, the fabrication of this circuit is complicated by the need to provide both digital and analogue components. This is particularly disadvantageous in the case where the drive circuit is to be fully integrated in the display panel and fabricated simultaneously with the components of the display panel using TFTs since analogue circuits using TFTs and offering adequate performance are generally more difficult to make.